It is proposed that IntraFascicular, Multielectrode Stimulation (IFMS), mediated by high electrode count microelectrode arrays, implanted intrafascicularly into peripheral nerves can significantly improve restoration of motion in paraplegic patients over that produced with cuff or epimysial electrodes. This proposition is based on the following advantages of IFMS. 1) Fewer implant sites are required to electrically activate many more muscles, greatly simplifying surgical procedures. 2) Because the tips of the implanted electrodes are in close proximity to the motor neurons, electrical currents required for stimulation are very low. 3) Each muscle is accessed by many individual electrodes, allowing more selective stimulation of individual muscles with low stimulation currents. 4) Each implanted electrode accesses an independent set of motor units which allows a more physiological recruitment of muscle force. 5) Access to independent motor units allows one to use more physiological stimulation protocols that greatly reduce muscle fatigue. Work is proposed to exploit these benefits of IFMS and to investigate new challenges encountered during IFMS restoration of stance behaviors in decerebrate cats and arising from the unprecedented access to the distal musculature provided by the Utah Slanted Electrode Array (USEA). The work will be performed in a hierarchical series of experiments and conducted within the context of a biomechanically realistic 3-D musculoskeletal model of the cat hind limb that will be developed in Specific Aim 1. The model will help specify which muscles to activate, and the forces in these muscles to achieve stance behaviors. Specific Aim 2 will develop stimulation algorithms that provide load-relevant, ripple-free, fatigue-resistant muscle forces. Specific Aim 3 will optimize the implantation sites of USEA's to access the maximum number of hip, knee, and ankle extensor muscles while providing ease of surgical implantation procedures. Specific Aim 4 will extend the isometric muscle force experiments (SA2-3) to understanding the dynamics of IFMS induced muscle contraction during physiological muscle trajectories. Specific Aim 5 will investigate the interactions that occur when torques are produced by multiple muscles at each joint. Specific Aim 6 will achieve the behaviorally relevant sit-to-stand and stance tasks in the acutely paralyzed cat model. These experiments will demonstrate the advantages of IFMS mediated by USEA's implanted in the cat femoral and sciatic nerves in achieving a set of multi-joint, multi-muscle, multi-limb motor behaviors relevant to the paraplegic population, and will form a prelude to future experiments on IFMS evoked gait in cats, and eventually, in humans.